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It’s inevitable that as we age, our brains get smaller. Nerves die off, losing their connections, and that leads to a thinned out network feeding our thinking functions. But brain shrinkage isn’t inevitable, according to the latest study of elderly adults.

In recent years, as more research reveals the benefits of staying both physically and mentally active into the golden years, health officials have been urging older people to exercise more and stay mentally engaged by maintaining a rich network of friends and family connections and by learning new skills to keep their brains sharp. Crossword puzzles, learning a new language, playing board games — anything that requires some thought can do the trick.

But researchers wanted to know whether physical activity or mental activity was better at keeping cognitive functions intact, or whether there was something about the combination that helped elderly stay sharp. So in the new study, researchers examined the medical records of 638 people from Scotland born in 1936. At age 70, the participants filled out questionnaires detailing their exercise habits as well as how often they engaged in stimulating mental and social activities. When they turned 73, the scientists took MRIs of their brains and matched their size, as well as any changes in the volume of white matter, which makes up the web of nerves that connect various brain regions, to the volunteers’ questionnaire answers.

The participants reported a range of physical activity, from household chores to heavy exercise or playing competitive sports several times a week. Over the three years, those who exercised the most had the largest brains, and showed the least shrinkage in white matter compared to those who were the least active, suggesting regular exercise in old age could protect the brain from age-related decline. Those who reported engaging in more intellectual pursuits didn’t show the same benefit.

“People in their seventies who participated in more physical exercise, including walking several times a week, had less brain shrinkage and other signs of aging in the brain than those who were less physically active,” said study author Alan J. Gow from the University of Edinburgh in Scotland in a statement. “On the other hand, our study showed no real benefit to participating in mentally and socially stimulating activities on brain size, as seen on MRI scans, over the three-year time frame.”

Previously, four studies presented at the Alzheimer’s Association International Conference in Vancouver also found that elderly adults who exercised regularly, including taking walks, working with light weights and aerobic training, had fewer “senior moments” and improved memory than those who were more sedentary.

Taken together, the findings contradict the traditional belief that older people inevitably become more frail physically, and less sharp mentally. The most recent studies also suggest that aging processes, including the decline in cognitive functions, isn’t permanent, but can be slowed by relatively moderate interventions such as walking.

How physical activity protects the brain still isn’t clear, but the researchers speculate that consistent exercise, particularly aerobic activities, promotes heart health, which in turn keeps nutrient-rich blood flowing to the brain to nourish neurons. People who are more physically active may also be healthier to begin with, both physically and mentally. “The possibility that physical activity is a proxy for better general health should not be overlooked,” the authors write. Either way, the latest findings, coupled with previous results, suggest that physical activity can be good for both the body and the brain, even into old age.

The study was published in n Neurology, the medical journal of the American Academy of Neurology.